Method of purifying uranium



Oct. 30, 1956 N. c. BEEsE 2,768,873

METHOD oF PURIFYING URANIUM Filed July 7, .1944

00 o /zao INVENTOR #05W/wv c. 55555 BY Wam ATTORN EY METHOD OF PURIFYING URANIUM Norman C. Beese, Verona, N. J., assignor to the United Stat-,es of America as represented by the United States Atomic Energy Commission Application July 7, 1944, Serial No. 543,893

6 Claims. (Cl. 2li-14.5)

This invention relates to a method for the purification of uranium, and more particularly to the reduction of the content of boron impurity in uranium.

A small percentage of boron impurity in uranium hinders the application of uranium to some uses, when it is desirable for the boron content to be .as low as possible. In some such uses of uranium, concentrations of boron of over .5 part per million are denitely objectionable. Uranium, of relatively low boron impurity content may be produced, for example, by the electrolysis of a bath composed of a halide of uranium fused in a mixture of chlorides held :at such a temperature that the metallic uranium is deposited as a coarse powder on a cathode of refractory material such as molybdenum. In the practice of this method potassium uranous `fluoride (called green salt) is first prepared by a photochemical reduction, `preferably using sunlight,.of a solution containing a uranium salt such as, for example, the nitrate, potassium fluoride, hydrofluoric acid, and areducing agent such as formic acid or sucrose; The po-V tassium uranous fluoride precipitate is removed, filtered, washed, and dried to yield 'a salt suitable for the" electrolysis operation. The green salt is then introduced into a bath of fused sodium and calcium chlorides in a graphite Crucible having a molybdenum electrode, and these are connected to a source of direct current so that the Crucible is the anode and the electrode is the cathode. The electrolysis is preferably carried out at a bath temperature of about 900 C. although lower and higher temperatures may be, and have been, used. The uranium metal which collectson the cathode is then removed, crushed, and ground, and thereafter washed and rinsed until the residue is of the desired fineness and substantially free of deleterious substances, such as salts Vfrom the electrolysis bath and finely divided metal and oxide.

However, uranium produced in accordance with this method may still contain an objectionable amount of boron impurity. If higher bat-htemperatures were employed the amount of boron impurity would be reduced in the product, but such temperatures give rise to practical difliculties such fas excessive corrosion of parts of the apparatus, and liquefying of the uranium at the cathode as the melting temperature of uranium is reached. Lower bath temperatures increase the amount of boron impurities in the uranium metal produced.

In those instances where the uranium metal produced is found to contain an objectionable amount of boron impurity for the contemplated use of the uranium metal, it is, of course, desirable that additional amounts of the boron impurity be removed, and in accordance with the present invention this may be accomplished effectively by burning the uranium Vto form Aan oxide thereof and then heating the uranium oxide to a temperature of about 1200" C. at which the boron impurities are substantially liberated from the oxide. The resulting purified oxide'then may be converted, for example to the niited States Patent* 2,768,873 Patented Oct. 30, 1956 trate and uranium metal again produced electrolytically as aforesaid without further increase in the boron content thereof. It has been possible by this procedure to produce uranium metal` having a boron impurity content as low as 0.5 part per millon.

It is accordingly an `object of the present invention to provide a method by which the boron impurity in uranium may be reduced to a very low figure.

Another object is the provision of such a method for the reduction of boron impurity in uranium which is relatively simple and economical to practice.

Still another object is to provide a method whereby uranium or its compounds may be made available with a low boron impurity concentration of the order of 0.5 part per million for uses in which Vlow boron content is necessary.

Other objects and advantages of the invention will be obvious herefrom or may be learned by practice with the'invention.

The accompanying d-rawing is a chart showing the relation of boron content to furnace temperature in the practice of the present invention.

In accordance with a preferred embodiment of the present invention, an oxide of uranium is produced by placing in a suitable receptacle uranium metal powder ture of -the oxdeis then raised inthe furnace to a temperature of about 1200 C. orl higher in an air `atmosphere and is preferably maintained at the elevated temperature until boron impurity has had an opportunity to volatilize. It has been found that the temperature is preferably maintained for about an hour when relatively small amounts of uranium oxide are used, as for example, about one gram samples. After removal from the furnace, the uranium oxide has a boron content of about 0.5 part per million, and'may be utilized wherever such boron free uranium is necessary.

The accompanying drawing illustrates the results obtained in tests made with about one gram samples of ground uranium oxide placed in the electric furnace,

held at the indicated temperatures for :about thirty minutes and thereafter analyzed for boron content. The furnace temperatures are plotted on the abscissa and range from room temperature to 1200 C., and the boron content is plotted on the ordinate in parts per million. It will be noted that the tests were run on samples without treatment in the electric furnace, and then in the electric furnace at 800, 900, 1000 and 1200 degrees centigrade. Analysis of the samples was made spectroscopically by a method involving the use of standard oxide samples calibrated chemically and spectroscopically at the Bureau of Standards.

j The two curves plotted in the drawing represent the behavior upon heating of two boron components found to be present in the uranium. Curve A represents the behavior of a boron component which will volatilize at temperatures below 1000 C. Vas shown in the drawing. This component was initially present in a concentration of 1.1 parts per million, and is reduced to about 0.6

Vpart per million by a temperature of 800 C. At 900 over 1100 C. do not decrease the concentration of said boron component which, at that temperature, is between 0.10 and 0.15 part per million.

The curve marked B in the drawing represents the behavior of another component of boron found to exist in the uranium. As shown, this component is initially present in concentration of about 1.3 parts per million, and moderate heating of the uranium oxide does not appreciably reduce the concentration. On heating the oxide to 1100., however the concentration of this component of the impurity is `reduced to about one part per million, and at 1200 C. the concentration has decreased to about 0.5 part per million. It seems reasonable to believe that the differences observed in the volatility of the two boron components are due to differences in the type of compounds involved, which might be variously a borate, a boride or a boron oxide. This explanation of the observed results, however, is advanced 'as a possibility only, and to no extent as a limitation of the present invention.

With the method of the present invention for reducing the boron impurity content of uranium by heating an oxide of uranium, it will be apparent that uranium metal having a very low boron concentration may be produced in a relatively simple and economical manner, and that uranium may thus be furnished for uses requiring such low boron concentrations.

This invention in its broader aspects is not limite-d to the specific processes and steps described but departures Y may be made therefrom within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

What I claim is:

1. A purification process which comprises oxidizing metallic uranium containing boron impurities at a Jternperature between about 400 and about 800 C. followed by heating the resultant uranium oxide containing the said impurities to a` temperature above 1000 C. to reduce the boron content thereof.

2. A purification process which comprises oxidizing metallic uranium containing boron impurities at a temperature between about 400 and about 800 C. in the presence of oxygen followed by heating the resultant oxidized material at a temperature of about 1200 C. to reduce the boron content thereof.

3. A purification process which comprises oxiding metallic uranium vcontaining boron impurities at an elevated temperature but substantially below 1000" C. followed by heating the resultant oxidized material at a temperature above 1000 C. thereby volatilizing the boron impurities and removing the same from the uranium oxide.

4. A purification process which comprises oxidizing metallic uranium containing boron impurities at an elevated temperature but substantially below 1000" C., comminuting the resultant oxidized mixture, heating the resultant comminuted oxidized mixture at a temperature above 1000 C. thereby volatilizing the boron impurities and removing the same from the oxidized uranium mass.

5. 'A purification process which comprises oxidizing metallic uranium containing boron impurities at an elevated temperature but substantially below l000 C., comminuting the resultant oxidized mixture, heating the resultant comminuted mixture at a temperature of about 1200 C. thereby volatilizing the boron impurities and removing the same from the oxidized uranium mass.

6. A purification process which comprises reacting metallic uranium containing boron impurities with oxygen at an elevated temperature but substantially below 1000 C., followed by heating of the resultant oxidized cornposition in the presence of oxygen at a temperature above 1000 C.

References Cited in the le of this patent Mellor: Inorganic and Theoretical Chemistry, vol. 12, pages 13 and 45. Longmans, London (1932). (Copy in Div. 59.) 

1. A PURIFICATION PROCESS WHICH COMPRISES OXIDIZING METALLIC URANIUM CONTAINING BORON IMPURITIES AT A TEMPERATURE BETWEEN ABOUT 400 AND ABOUT 800* C. FOLLOWED BY HEATING THE RESULTANT URANIUM OXIDE CONTAINING THE SAID IMPURITIES TO A TEMPERATURE ABOVE 1000* C. TO REDUCE THE BORON CONTENT THEREOF. 